There is oxygen, carbon dioxide and nitrogen in synovial fluid. When you turn your neck, pressure against the synovial fluid drops and a bubble of the carbon dioxide forms and pops as it comes off the synovial fluid. It pops too when it bursts causing a popping sound when you turn your neck. A cracking sound may be heard as the ligaments move over uneven bone surface. A thorough explanation that will help you understand your position is given on http://sportscenteraustin.blogs.com/the_view/2005/03/neck_noises.html. Consider this reading too; http://www.spinegroup.com/Advice/NkCrk.html.